In a typical radio communications network, also known as a cellular radio system or a wireless communication network, wireless devices, also known as mobile stations, wireless terminals and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations. Transmissions towards the base station from the wireless device is called uplink (UL) transmissions, and transmissions towards the wireless device from the radio base station is called downlink (DL) transmissions.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for wireless devices. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base stations without reporting to RNCs.
In modern cellular radio systems, the radio access network has a strict control on the behavior of the wireless device. Uplink transmission parameters like frequency, timing, and power are regulated via downlink control signaling from the base station to the wireless device.
At power-on or after a long standby time, the wireless device is not synchronized in the uplink transmissions. The wireless device can derive an uplink frequency and power estimate from e.g. the downlink control signals. However, a timing estimate is difficult to make since the round-trip propagation delay between the eNodeB, radio base station in LTE, and the wireless device is unknown. So even if wireless device uplink timing is synchronized to the downlink, UL transmissions may arrive too late at the eNodeB receiver because of the propagation delays. Therefore, before commencing traffic, the wireless device has to carry out a Random Access (RA) procedure to the RAN. After the RA or actually during RA procedure although after a RA preamble sequence transmission, eNodeB can estimate the timing misalignment of the wireless device uplink and send a correction message. During the RA, uplink parameters like timing and power are not very accurate. This poses extra challenges to the dimensioning of a RA procedure.
Usually, a Physical Random Access Channel (PRACH) is provided for the wireless device to request access to the network. A RA preamble is used which is based on a specific sequence with good correlation properties e.g. good auto-correlation. Because multiple wireless devices may request access at the same time, collisions may occur between requesting wireless devices. A contention resolution scheme has to be implemented to separate the wireless device transmissions. To distinguish between different wireless devices performing RA typically many different RA preambles exist. A wireless device performing RA randomly picks a RA preamble out of a pool of RA preambles and transmits the picked RA preamble. The RA preamble represents a random wireless device ID which may be used by the eNodeB when granting the wireless device access to the network. The eNodeB receiver may resolve RA attempts performed with different RA preambles and send a response message to each wireless device using the corresponding random wireless device IDs. In case that multiple wireless devices simultaneously use the same RA preamble a collision occurs and most likely the RA attempts are not successful since the eNodeB cannot distinguish between the two wireless devices with the same random wireless device ID.
To minimize the probability of collision the set of available sequences, i.e. RA preambles, should be large. In LTE the number of provided sequences per cell and RA opportunity is 64.
RA preambles assigned to adjacent cells are typically different to insure that a RA in one cell does not trigger any RA events in a neighboring cell.
LTE defines different RA configurations that differ in the amount of offered RA opportunities. A RA opportunity is approximately 1 MHz wide and either 1, 2, or 3 ms long within which the wireless device may transmit the RA preamble. In the configuration with the lowest number of opportunities one RA opportunity is offered every second radio frame, i.e. every 20 ms. On the other extreme the configuration with the highest density of RA opportunity offers one RA opportunity every subframe, i.e. every ms.
An eNodeB receiver listens at all RA opportunities to detect RA preambles. In case a RA preamble is successfully detected a RA response that includes an identifier or a number of the detected RA preamble is sent in a special message on the DL. A wireless device that has recently performed a RA attempt is listening within a certain time window after the RA preamble has been sent on the DL to receive a RA response. In case of a successful reception of the RA response the wireless device continues with steps of the RA procedure for contention resolution. In case no RA response is received within the specified window a new attempt is made. Also, if the contention resolution does not indicate that the wireless device won the contention, a new attempt is made. The power of this new RA preamble transmission is increased by a configured step size relative to the previous attempt. Depending on the back-off parameter in the wireless device, the wireless device may immediately re-try or wait for a random time depending on the configured back-off time prior a new attempt.
In addition to a contention-based RA procedure, LTE supports a contention-free variety of the RA procedure in which eNodeB directs the wireless device to use a specific RA preamble not simultaneously used by any other wireless device in the same cell and the steps of contention resolution are not needed.
If the number of unsuccessful RA attempts exceeds a configured threshold, lower layers in the wireless device indicate “random access problem” to higher layers. In e.g. LTE, Medium Access Control (MAC) layer indicates random access problem to Radio Resource Control (RRC) layer and continue random access attempts. Depending on higher layer state and conditions, higher layers declare radio link failure unless some higher layer procedure timer(s) are running or let the random access procedure continue until one or more higher layer procedure timers expire/time-out or a stopping condition is met.
E.g., to allow opportunity to recover from temporary radio problems and to accommodate various delays, e.g., for processing, inter-node communication etc, the duration of some higher layer procedures, including connection establishment, handover, radio link monitoring and connection reestablishment, are governed by timers. Durations of the timers may be configurable. Such timers in LTE comprise timer T300, related to connection establishment; timer T304, related to handover; timers T301 and T311, related to connection re-establishment; and timer T310, related to radio link monitoring.
Upon declaration of radio link failure (RLF), or upon time-out or stopping condition is met for the higher layer procedure, RRC layer resets lower layers, thereby aborting the ongoing random access procedure.
A connection re-establishment procedure provides an optimized means for recovering a connection after RLF and/or handover failure. The connection re-establishment procedure reduces the signalling compared to requesting a new connection by means of a connection establishment procedure. However, a problem of prior art is that it is the random access procedures during e.g. re-establishment procedures consume a rather large amount of radio resources, resulting in a rather large consumption of radio resources when accessing the wireless communications network.